1. Field of the Invention
The present invention relates to an audio apparatus for measuring acoustic characteristics.
2. Description of the Related Art
The impulse response between a sound generation source and sound receiving points in an acoustic space, such as a room or a hall, includes important information about acoustic characteristics of the acoustic space. The impulse response measured in a well-known hall, for example, can be stored in a storage unit of an audio apparatus. By performing filtering processing to convolve the impulse response signal into a signal of a musical piece to be played back, acoustic effects can be produced which will make a user feel as if the user were listening to music in the real hall. Similarly, when a microphone is placed at a listening point where the user sits in the room and measuring signals are emitted from speakers, a room impulse response between the listening point and the speakers can be measured. The measured room impulse response is used to generate a sound field correction filter. The sound field correction refers to processing for flattening the irregularity of amplitude-frequency characteristics of the impulse response caused by interference between direct sound and reflected sound in the room, particularly, flattening peaks and dips induced by low-frequency standing waves, which have considerable effects on the auditory sensation. In addition, a clear sound image can be obtained by performing delay compensation. The delay compensation makes coincident the rise start times regarding the impulse response between the respective speakers and the listening point.
As described above, it is useful to obtain the impulse response in various kinds of acoustic processing in the audio apparatus. In this respect, what counts is a technique to measure the impulse response with high accuracy, in other words, a technique to suppress the influence of noise on measurement of the impulse response.
When the impulse response is measured in a room, since there is intrinsic background noise in the room, the signal-to-noise ratio of a signal picked up by the microphone deteriorates. For this reason, Japanese Patent Application Laid-Open No. 2002-330500 discusses a technique which measures an environment noise level and determines a level of the measuring signal in order to secure a better signal-to-noise ratio in relation to the background noise.
It is a common practice to generate a measuring signal a plurality of times to obtain a single impulse response and perform synchronized addition (=signal averaging) of sound collection signals to cancel out the background noise to improve the signal-to-noise ratio.
Japanese Patent Application Laid-Open No. 2005-346815 discusses a method of removing irregular noise from burst signals of plural periods read by a read processing unit of the disk device. More specifically, integrated values of absolute values of period burst signals are compared and a predetermined number of periods counted from a maximum value and/or a minimum value are not used for subsequent processing. In this manner, noise is removed from the signals.
In measurement of the room impulse response, while a sound is being collected, besides steady background noise, such as a running air conditioner, unexpected sudden or transient noise is liable to occur, such as a ringing telephone, human voice, an opening or closing door, and a honking from a car outside. In audio apparatuses these days, multi-channel speakers have been adopted and the measuring points have increased due to the gradual expansion of a listening area for which the sound field correction is required. The required number of times of measurement of impulse response signals is on the increase. There is a high possibility that the sudden noise described above is mixed in the sound collection signal during measurement.
According to the method discussed in Japanese Patent Application Laid-Open No. 2002-330500, the sound-to-noise ratio relative to steady-state environment noise can be improved, but this method fails to offer countermeasures against the sudden noise. When a clipping of the sound collection signal is detected, if simple measures, such as re-measurement, are taken, only sudden noise of a very high level can be determined, and re-measurement takes additional time.
The method discussed in Japanese Patent Application Laid-Open No. 2005-346815 can be used to remove sudden noise from the sound collection signal. However, out of signals of plural periods, the number of unusable periods has already been set as a default value, and noise countermeasures do not take the status of a signal transmission system into consideration and can never be viable measures for noise reduction. Therefore, there is a possibility that usable periods not including noise are not used for subsequent processing and periods including noise are used for subsequent processing.